Devices for transfer of particulate and/or fibrous material into pressurized reactors are of interest in conversion of lignocellulosic biomass such as straw, grasses, corn stover, bagasse and waste wood to bioethanol and/or other useful products.
In order to provide hydrothermal pre-treatment of lignocellulosic biomass at the scale of commercial bioethanol production, throughput of 10-40 tons/hour will often be required through reactors pressurized to 10-40 bar. To achieve these high levels of “feeding” capacity, devices capable of continuous transfer of feedstocks from low pressure into pressurized reactors will be desirable.
In order to reduce energy consumption of hydrothermal pretreatment processes, it is advantageous to maintain low water content, that is, to process feedstocks at a high dry matter concentration. For hydrothermal pre-treatment of lignocellulosic biomass, more than 15% dry matter is usually considered a high dry matter content, with 30-45% dry matter as a preferred range.
The very low specific density of most lignocellulosic biomass (around 50 kg/m3 for chopped wheat straw and corn stover) makes compaction of the feedstock during feeding advantageous. Many particulate and for fibrous materials, such as straw, bagasse and household waste, also require force-loading because they have poor flow qualities, and are inclined to form bridges.
One attractive solution for transfer of particulate and/or fibrous materials into pressurized reactors is the “plug feeder.” Plug feeders use loading devices to compact particulate and/or fibrous material to a high density “plug” that provides a pressure seal, because it can not be penetrated by gasses. Plug feeders can work continuously at high dry matter concentration and further provide force-loading of materials into the reactor. The density necessary to provide sufficient sealing properties of the plug may vary depending on the feedstock and the moisture content. With cereal straw, plug densities typically correspond to a specific density of 700-1100 kg solid dry matter per m3.
The plug feeder art faces several challenges that are the subject of continuing innovation. One challenge is to provide a plug having sufficient sealing properties to provide a secure barrier to the high reactor pressure. Another challenge is to avoid wear, caused by friction from the high density plug. This problem is especially great with plugs formed from particulate and/or fibrous material having a high silica content, such as rice straw, wheat straw and corn stover. Another challenge is the safety of pressure sealing in commercial scale operations. Even if a plug is normally sufficiently pressure tight, heterogeneity of feedstock material may from time to time result in formation of channels with inferior sealing properties. This can cause an explosion-like situation, when pressure in the reactor is suddenly released. This not only causes a production stop, but is potentially dangerous to personnel. Another challenge is the requirement for disintegration of the plug at the entrance to the high pressure reactor. Still another challenge is the general requirement to minimize energy consumption and processing steps. Friction between the high density plug and the feeding equipment may also cause such high temperatures that the feedstock is thermally degraded, and therefore less suited for further treatment.
In the prior art, the primary challenge of pressure sealing has generally been addressed by increasing plug density (ref 5) or by trying to prevent negative effects of high plug density (ref 3). These “high density” solutions have disadvantages, however. High density plug feeders often require mechanical degradation (particle size reduction) of feedstocks in order to produce a plug that is gas impenetrable. Particle size reduction requires additional process steps and high energy consumption. High density plugs also introduce greater wear on machinery and require greater efforts at disintegration. High density plugs also result in higher energy consumption in that these require a greater degree of compaction and compensation for friction against the walls and other parts of the feeder. High density plugs also require more extensive disintegration at the reactor inlet, sometimes under harsh conditions. Moreover, plug density is only an indirect measurement of pressure sealing properties. One plug can have good sealing properties at low density and another plug can have bad sealing properties at high density.
Here we describe a new, “low density” approach to feeder methods and devices that provide transfer of a particulate and/or fibrous material from a zone 1 with a lower temperature and pressure into a zone 2 with a higher temperature and pressure. Vapour is allowed to penetrate into a comparatively low density plug in a controlled manner. Vapour condensation in the compacted feedstock eliminates leakage of vapour from zone 2 to zone 1. Embodiments of methods and devices that utilize this new approach are referred to as “flow feeders.” Flow feeder methods and devices described here provide general advantages of reduced wear, reduced energy consumption and reduced need for plug disintegration at the inlet to zone 2. In many cases, plug disintegration is not necessary at all. Preferred embodiments provide improved operational safety, improved capacity to work with heterogeneous feedstocks, and improved capacity to work with feedstocks having long particles, such as straws and grasses, or large particles, such as household waste.